Jose Mourinho has admitted Henrikh Mkhitaryan may not feature against Rostov due to the state of the pitch in Russia.
The Armenian limped out of United's second leg last-16 tie against St Etienne and hasn't featured since, missing the EFL Cup final win against Southampton.
He is part of the travelling squad that has made the long journey, but Mourinho says his hand may be forced by the tricky surface.
“It's still hard for me to believe we are going to play tomorrow,” said Mourinho. “I know we have to, but it is hard for me to believe we are going to play tomorrow in that field - if you can call it a field.
“And I don't know what team to play, really. I don't know if Henrikh Mkhitaryan is going to play.
“I don't know. I have now a lot to think because I was expecting something more playable in a vert similar pitch in the summer in China we and Manchester City decided not to play, but it looks like we have to play.
“I didn't have any contact with anyone until now [about the pitch].
"I just had a quick contact with one gentleman from UEFA when I was looking at the pitch and when I told him about my concerns, the gentleman told me the players are insured, so if something happens, no problem.”
Asked if there was anything United could do in terms of the game going ahead or not, Mourinho said: “I don't know, I really don't know. I was analysing these very good opponents that we didn't speak about yet.
“In their previous matches against Bayern Munich, Ajax, Atletico Madrid, in all these matches it looked for me a very playable pitch and I was very surprised with what I found here today.”
On Zlatan Ibrahimovic's three-match domestic ban for violent conduct, Mourinho said: “I have no reaction He plays tomorrow and I don't comment abut the domestic decision.”